It is widely proposed that the influence of stress on immune function is the primary biological pathway linking stress to increased infectious disease susceptibility. However, the literature is missing empirical evidence for this pathway, in part because of a failure to focus on immune measures that are relevant for the development of host resistance. The broad objective of the proposed study is to identify pathways linking psychological stress to in vivo immune processes that are clinically relevant for the development and maintenance of resistance to infectious disease. For this purpose, a hepatitis B vaccination model is employed. This model permits the systematic exploration of behavioral and biological pathways linking stress to ability to mount and maintain primary and secondary antibody responses following exposure to a novel antigen. It also allows the exploration of the role that individual differences in the magnitude of biological responses to stress play in vulnerability to stress-immune relationship. In this regard, it has been demonstrated that individuals vary consistently in the magnitude of their cortisol and immune responses to acute stress, conceivably rending them more or less able to mount an antibody response to vaccination. One hundred and eight-two healthy males and females aged 40-60 years will be recruited to participate in the proposed longitudinal research, which includes a laboratory based physiological reactivity phase, a prospective surveillance phase and a follow-up period. In the reactivity phase, participant's immune and cortisol responses to an acute laboratory stressor will be measured to determine individual differences in the magnitude of immunologic reactivity to stress. During this phase, participants will also complete self-report measures of recent life stress and trait negative affect. In the surveillance phase, participants will be administered the three does of hepatitis B vaccine by standard protocol. They will also complete daily stress diaries for the seven days surrounding each dose of vaccine to assess levels of acute stress. Saliva samples for the assessment of cortisol levels will be collected at the time of each vaccination. Antibody responses will be measured at the time of the second and third does of vaccine and, during the follow-up period, 6 and 12 months following completion of the vaccination series.